The invention relates to a method for operating a gas scrubber in which components are removed from a crude gas by scrubbing with a chemical or physical scrubbing medium to obtain a pure gas present at elevated pressure which, after heating, is introduced into an expansion machine which after work-performing decompression the pure gas leaves at an exit temperature.
A process of the type in question is known for example from patent specification U.S. Pat. No. 3,824,766. Therein, natural gas is subjected to a scrubbing operation with a physical carbon-dioxide-saturated scrubbing medium to remove sulphur components. The pure gas obtained from the scrubber is initially heated against to-be-scrubbed natural gas before it is subjected to work-performing decompression in an expansion turbine and thus cooled. In a downstream scrubbing apparatus the cold pure gas is subjected to a scrubbing operation with carbon-dioxide-free scrubbing medium, which is thus cooled and saturated with carbon dioxide.
Patent application EP0707880 likewise discloses a process for gas separation where, after heating against to-be-scrubbed crude synthesis gas, a synthesis gas obtained by methanol scrubbing is decompressed in an expansion turbine and thus cooled. The thus obtained cold is utilized in the process for cooling process streams.
As a result of the work-performing decompression in the turbine, energy which may be introduced into the electrical system of the plant or otherwise utilized is obtained in each case.
In principle a large pressure difference between the entry side and the exit side of the expansion machine is advantageous since this increases the recovered energy amount and also lowers the temperature of the expanded pure gas so that it may be more advantageously used for cooling purposes. However, low temperatures are problematic when components of the pure gas freeze out and form solids, since these can damage a turbine employed as an expansion machine in particular. Even when the temperature of the expanded pure gas is sufficiently high to safely avoid freezing-out of components in the expansion machine, the cold gas cannot usefully be utilized for cooling purposes when its use results in solids formation in a material stream to be cooled. The temperature of the pure gas below which solids formation takes place is referred to as the critical temperature.
It is therefore necessary to limit the exit temperature to a value above the critical temperature. The simplest way to achieve this is to not fully utilize the pressure difference available for the decompression via the expansion machine and to limit downward the pressure with which the pure gas leaves the expansion machine. However, the disadvantage of this is that both the energy obtainable in the expansion machine and the amount of cold from the decompressed pure gas utilizable in the process are reduced.